The present invention generally relates to the processing of fly ash. In particular, the present invention relates to a method and apparatus for combusting and reducing residual carbon in fly ash.
Coal is still today one of the most widely used fuels for the generation of electricity, with several hundred power plants in the United States alone and an even greater number worldwide, utilizing coal combustion to generate electricity. One of the principal by-products from the combustion of solid fuels such as coal is fly ash, which generally is blown out of a coal combustor and contained within the exhaust air stream coming from the combustor. Fly ash has been found to be very useful in building materials applications, particularly as a cement additive for making concrete, due to the nature of ash as a pozzolanic material useful for adding strength, consistency and crack resistance to the finished concrete products.
Most fly ash produced by coal combustion, however, generally contains a significant percentage of fine, unburned carbon particles, sometimes called xe2x80x9ccharxe2x80x9d, that reduces the ash""s usefulness as a byproduct. Before the fly ash produced by the combustion of coal and/or other solid fuels can be used in most building products applications, such as for a cement additive for concrete it must be processed or treated to reduce residual carbon levels therein. Typically, it is necessary for the ash to be cleaned to as low as 1-2 percent carbon content or less before it. can be used as a cement additive and in other building products applications. If the carbon levels of the fly ash are too high, the ash is unacceptable for use. For example, fly ash production in the United States for 1998 was in excess of 55 million tons. However, less than 20 million tons of fly ash were used for building product materials or other purposes. Carbon content of the ash is thus a key factor retarding its wider use in current markets and the expansion of its use to other markets.
In order to remove the residual carbon from fly ash to such low levels, it generally is necessary to ignite and combust the carbon out of fly ash. This requires that the fly ash particles be supplied with sufficient temperature, oxygen and residence time in a heated chamber to cause the carbon within the fly ash particles to ignite and burn, leaving clean ash particles. Currently, a number of technologies have been explored to try to effect carbon combustion in fly ash to reduce the carbon levels as low as possible. The primary problems that have faced most commercial methods in recent years generally have been the operational complexity of such systems and maintenance issues that have increased the processing costs per ton of fly ash processed, in some cases, to a point where it is not economically feasible to use such methods.
Such current systems and methods for carbon reduction in fly ash include, for example, the system disclosed in U.S. Pat. No. 5,868,084 of Bachik in which the ash is conveyed in basket conveyors and/or on mesh belts through a carbon bum out system that includes a series of combustion chambers. As the ash is conveyed through the combustion chambers it is heated to burn off the carbon therein. Other known ash feed or conveying systems for transport of the ash through combustion chambers have included screw mechanisms, rotary drums and other mechanical transport devices. At the high temperatures typically required for ash processing, however, such mechanisms have often proved difficult to maintain and operate reliably. In addition, such mechanisms typically limit the exposure of the carbon particles to free oxygen by constraining or retaining the ash within baskets or on mesh belts such that combustion is occasioned by, in effect, diffusion through the ash, thereby retarding the effective throughput through the system. Accordingly, carbon residence times within the furnace also must be on the order of upwards of 30 minutes to affect a good burn out of carbon, all of these factors generally resulting in a less effective and costlier process.
Another approach to generating carbon combustion in fly ash has utilized bubbling fluid bed technology to affect carbon bum out, as disclosed in U.S. Pat. No. 5,160,539 of Cochran, et al. In this system, the ash is placed in a bubbling fluid bed supplied with high temperature and oxygen so that the carbon is burned or combusted as it bubbles through the bed. This bubbling fluid bed technology generally requires residence times of the carbon particles within a furnace chamber for up to about 20 minutes or more. The rate of contact the carbon particles with oxidizing gasses in the bubbling fluid bed also is generally limited to regions in which the bubbles of gas contact solids such that the rate of contact is related to the effective gas voidage in the bubbling bed, which is typically around 55-60 percent (i.e. around 40-45 percent of solids by volume). These systems have, however, been found to have limited through-put of ash due to effective carbon combustion rates with required carbon particle residence times generally being close to those of other conventional systems.
Accordingly, it can be seen that a need exists for a method and apparatus for processing fly ash to sufficiently clean the ash of residual carbon that addresses these and other related and unrelated problems in the art.
Briefly described, the present invention comprises a method and system for processing fly ash particles to combust and reduce levels of residual carbon within the fly ash. The system and method of the present invention is designed to optimally expose the fly ash to oxygen and temperature at sufficient levels, and with sufficient residence time, to cause combustion of residual carbon within the ash to substantially reduce the levels of carbon remaining in the ash.
The combustion system generally includes a reactor having an inlet, or first end, and a second, outlet or exhaust end, with a reactor chamber being defined within the reactor. The fly ash is initially received within the reactor chamber in a dense phase particulate bed composed of fly ash particles or a combination of fly ash particles and an inert particulate material. Typically, the inert particulate material will be a coarse particulate such as silica or alumina sand, or other inert oxide materials that have a sufficient size and density to remain in the particulate bed as an airflow is passed therethrough. A heat source is generally positioned within or around the reactor or adjacent the particulate bed for heating the bed and the reactor chamber to a temperature sufficient to ignite and combust the carbon of the fly ash. A motive air source further generally is provided adjacent or with the heat source for supplying a heated flow of air through the reactor chamber.
As the fly ash within the particulate bed is subjected to entraining forces from the heated airflow, the fly ash particles generally are caused to migrate through the particulate bed. The particulate bed provides a larger thermal mass for heat exchange between the fly ash particles and helps promote greater residence time of the fly ash within the reactor chamber to promote ignition and combustion of the residual carbon. The combustion of the carbon of the fly ash is continued as the fly ash particles are passed from the particulate bed and are conveyed through an upper region of the reactor chamber in a dilute suspension or phase, entrained within the heated air flow, toward the outlet of the reactor. While being conveyed in this dilute phase through the upper region of the reactor chamber, the fly ash particles are further exposed to oxygen to enhance the combustion of carbon from the fly ash.
The fly ash particles thereafter are exhausted with the airflow to a primary or recirculated ash capture. The recirculated ash capture generally is a separator, such as a cyclonic separator, having an inlet connected to the reactor, an air exhaust, and an outlet at its opposite end. The fly ash is separated from the air flow in the ash capture, with the air being exhausted, typically to a secondary ash capture, filtration system, or other downstream processor or system for further filtering or cleaning of ash from the exhaust air flow. The fly ash separated from the airflow in both the recirculated ash capture and secondary ash capture generally is collected for dispensing to an ash feed accumulator. It is also possible to provide a raw material feed connected to the recirculated ash capture for feeding raw, unprocessed fly ash into the system. Alternatively, the raw material feed can be connected directly to the reactor for feeding raw, unprocessed ash directly to the particulate bed within the reactor chamber, or to the ash feed accumulator for mixing or combining with recirculated fly ash for injection into the particulate bed.
The ash feed accumulator generally includes a collection vessel such as a stand-pipe or other device, connected to the outlet of the recirculated ash capture and to the inlet of the reactor by a injector pipe or conduit. The ash feed accumulator receives recirculated, processed fly ash from the recirculated ash capture, and possibly from the raw material feed in some embodiments, and collects and compiles the fly ash in an accumulated bed. The accumulator typically is aerated to maintain a desired pressure in the accumulator bed, so as to create a head of solids for injection of fly ash into the particulate bed. The hydrodynamic force of the head pressure acting within this accumulator bed urges the fly ash particles through the injection pipe to provide a feed or flow of fly ash to the particulate bed. As a result, as the level of fly ash accumulated within the accumulator bed increases to a level where its head pressure is in excess of the back pressure exerted on the injector conduit by the particulate bed, fly ash is injected from the ash feed accumulator into the particulate bed of the reactor.
The system of the present invention thus provides for recirculation of the fly ash through the combustor system as needed to combust and substantially remove carbon from the fly ash particles. Once sufficiently cleaned. of carbon, the fly ash can then be dispensed from the combustor system for collection and cooling.
Various objects, feature and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description, when taken in conjunction with the accompanying drawings.